1. Field of the Invention
This invention relates to a pressure sensor that introduces a pressure from a pressure introduction portion to a sensing portion accommodated in a case and is fitted, at this pressure introduction portion, to a measured member through a seal ring. The invention can be applied to a pressure sensor for detecting a pressure inside an EGR pipe in an EGR system, for example.
2. Description of the Related Art
Restrictions on emissions have been tightened in recent years and NOx and PM (particulate matters) produced by a Diesel engine must be reduced. An EGR (Exhaust Gas Recirculation) system has drawn an increasing attention, particularly, as a system for reducing NOx.
The system returns a part of an exhaust gas to an intake side and lowers an oxygen concentration to reduce NOx. In this system, it is important to control the return quantity from the exhaust side to the intake side, and a pressure sensor is used as means for detecting a flow rate.
A pressure sensor including a sensing portion for detecting a pressure, a case for accommodating the sensing portion and a pressure introduction portion for introducing pressure from outside the case to the sensing portion has been proposed as such a pressure sensor (refer to Japanese Unexamined Patent Publication No. 2002-221462, for example).
When such a pressure sensor is assembled to the EGR pipe as the measured member, the pressure sensor is generally fitted to the EGR pipe through a ring-like seal ring (an O-ring) at the pressure introduction portion.
FIG. 9 is a schematic sectional view showing an ordinary example of an assembly process of the pressure sensor 900 to the EGR pipe 200. FIGS. 10A to 10C show the fitting surfaces (that is, seal surfaces) of the pressure introduction portions 12b and 13b in the pressure sensor 900 shown in FIG. 9 and the shape of the seal ring (O-ring) 130.
Here, FIG. 10A is a sectional view of the pressure introduction portions 12b and 13b. FIG. 10B is a plan view of the fitting surfaces 12c and 13c. FIG. 10C shows a dimensional relation between a groove 140 formed in the fitting surface 12c, 13c and the seal ring 130.
In this pressure sensor 900, a sensing portion (not shown) for detecting a pressure is accommodated in a case 10 as shown in FIG. 9. Pressure introduction portions 12b and 13b having openings are formed in the case 10. The pressure sensor 900 is fixed to an EGR pipe 200 by screw-fastening, etc., through a bracket 110.
The pressure introduction portions 12b and 13b are fitted to the EGR pipe 200 through the seal rings 130 made of an elastic material such as rubber. Consequently, pressures P1 and P2 from inside the EGR pipe 200 are introduced without leakage into the case 10 through the openings of the pressure introduction portions 12b and 13b and are guided to the sensing portion described above.
As shown in FIGS. 10A to 10C, a ring-like groove 140 for accommodating the seal ring 130 is formed in the fitting surface 12c and 13c of each pressure introduction portion 12b and 13b. 
To measure the flow rate of the EGR, it is customary to dispose an orifice 210 inside the EGR pipe 200 and to detect a pressure difference between portions ahead of and behind the orifice 210 as shown in FIG. 9. In other words, the pressure sensor 900 shown in FIG. 9 is constituted as a pressure difference (relative pressure) detection type pressure sensor 900.
Incidentally, the internal construction of the EGR pipe 200 at the connection portion with the pressure sensor 900 includes an upstream side pressure discharge path 220 for discharging an upstream side pressure P1 of the orifice 210 and a downstream side pressure discharge path 230 for discharging a downstream side pressure P2 of the orifice 210 as shown in FIG. 9.
The upstream side pressure discharge path 220 communicates with one of the pressure introduction portions 12b of the pressure sensor 900 through the seal ring 130 and the downstream side pressure discharge path 230 communicates with the other of the pressure introduction portions 13b of the pressure sensor 900 through the seal ring 130.
As for the detection pressure, a pulsation peak of the exhaust pressure reaches maximum of 300 kPa depending on the exhaust quantity and on the absence/existence of a turbo. Therefore, ordinary rubber hose piping cannot be used for the connection of the pressure sensor and the EGR pipe, and an assembly structure using a seal ring 130, that is, a so-called “direct mount structure”, is employed as shown in FIG. 9.
According to this direct mount structure, however, the seal ring 130 faces down when the pressure sensor 900 is set to the mounting portion of the EGR pipe 200. Therefore, in the ordinary O-ring seal structure in which an outer diameter D1 of the seal ring 130 is smaller than an outer diameter D2 of the groove 140 as shown in FIG. 10C, the seal ring 130 falls off from the groove 140.
To prevent the seal ring 130 from falling off (“fall-off”) at the time of assembly to the measured member, a structure using a seal ring 130 having a rectangular sectional shape has been employed in the past as shown in FIG. 11A.
FIGS. 11A to 11C show a press-in structure using the seal ring 130 having the rectangular sectional shape of the prior art into the groove 140. FIG. 11A shows a dimensional relation between the seal ring 130 and the groove 140. FIG. 11B shows a state after the seal ring 130 is pressed into the groove 140. FIG. 11C shows the state after the pressure sensor 900 is assembled to the measured member 200.
As shown in FIG. 11A, the prior art employs the structure in which the seal ring 130 has the rectangular section and the outer diameter of the overall seal ring 130 is greater than the outer diameter of the groove 140.
Consequently, the state where the seal ring 130 is pressed as a whole into contact with the groove 140, that is, the structure in which the seal ring 130 is gently pressed as a whole into the groove 140, can be established, so that the seal ring 130 is held by the groove 140 and fall-off is prevented.
According to this structure, however, there is the possibility that the distal end of the seal ring 130 swells out transversely from the groove 140 when the seal ring 130 is pressed into the groove 140 as shown in FIG. 11B.
Then, the distal end portion of the seal ring 130 is clamped between the fitting surface 12c, 13c and the mounting portion of the measured member 200 as shown in FIG. 11C. In other words, the seal ring 130 swells out (“swell-out”) and the seal ring 130 is damaged.
The problem described above is a common problem in pressure sensors employing the direct mount assembly structure in which the pressure introduction portion is fitted to the measured member through the ring-like seal ring and in the pressure sensor assembled to the EGR pipe as the measured member.